Optical properties of highly excited direct gap semiconductors

The electronic excitations in direct gap semiconductors interact strongly with the photon field. We discuss both the experimental and the theoretical aspects of the optical properties of these materials under strong optical excitation. We distinguish between intermediate excitation levels at which the electronic excitations form a dense system of excitons and excitonic molecules and very high excitation levels at which a degenerate electron-hole plasma occurs. The optical spectra of dense excitonic systems, which are mainly observed in copper halides and II–VI compounds, are shown to be determined mainly by the interaction processes between excitonic molecules, polaritons and free carriers. The optical properties of the electron-hole plasma, which has been observed in II–VI and especially in III–V compounds, can be understood only by taking into account many-body effects, such as dynamical screening of the Coulomb interactions, plasmon-assisted transitions and excitonic enhancement.     

Picosecond studies of highly excited CdS

Results on picosecond luminescence and excite-and-probe transmission as well as transient grating measurements for highly excited CdS measured at a bath temperature of 5 K will be presented. The luminescence and optical gain both due to electron-hole plasma and excitonic molecule recombination are observed. The electron-hole plasma decays very fast by bimolecular recombination of electrons and holes in the plasma and diffusion of the carrier toward the low density regions, and transforms into excitons and excitonic molecules within 100–200 ps. The possibility of electron-hole liquid formation is definitely excluded. The exciton and excitonic molecule decay rather slowly and govern the optical properties for times longer than 200 ps.     

Microscopic theory of optical excitations,photoluminescence, and terahertz response in semiconductors

This article presents a comprehensive many-body theory for optically excited semiconductors. The coupled equations of motion for the correlation functions of the Coulomb-interacting electron-hole system are derived and solved for different excitation conditions. The generation of a coherent excitonic polarization and its conversion into incoherent populations is analyzed. The spontaneous emission properties of the excited system are evaluated using a fully quantized theory. Luminescence from excitonic and electron-hole plasma populations is computed, and significant hole burning in the exciton center of mass distributions is predicted. It is shown how different excitations states of the many-body system can be identified by their characteristic signatures in the absorption spectra of a terahertz probe field.     

From Si nanowires to porous silicon: the role of excitonic effects

We show that the electronic and optical properties of silicon nanowires, with different size and orientation, are dominated by important many-body effects. The electronic and excitonic gaps, calculated within first principles, agree with the available experimental data. Huge excitonic effects, which depend strongly on wire orientation and size, characterize the optical spectra. Modeling porous silicon as a collection of interacting nanowires, we find an absorption spectrum which is in very good agreement with experimental measurements only when the electron-hole interaction is included.     

Excitonic condensation in a symmetric electron-hole bilayer

Using diffusion Monte Carlo simulations we have investigated the ground state of a symmetric electron-hole bilayer and determined its phase diagram at T = 0. We find clear evidence of an excitonic condensate, whose stability however is affected by an in-layer electronic correlation. This stabilizes the electron-hole plasma at large values of the density or interlayer distance, and the Wigner crystal at low density and large distance. We have also estimated pair correlation functions and low-order density matrices to give a microscopic characterization of correlations as well as to try and estimate the condensate fraction.     

Excitons in highly optically excited gallium phosphide

A study has been made of luminescence in weakly (10¹⁵-10¹⁶ cm^-3) and heavily (10¹⁸-10¹⁹ cm^-3) N-doped GaP crystals induced by 1.78, 2.34 and 3.56 eV photons from Q-switched ruby or neodymium lasers with a KDP crystal for second harmonic generation. The results which were obtained at excitations up to 10²⁰ cm^-3 electron-hole pairs are interpreted as the transitions: single bound excitons, bound excitonic molecules, free excitons in weakly-doped GaP, as well as Auger processes and the formation of a new excitonic state similar to a solid metal with high density of single excitons and excitonic molecules bound to isoelectronic traps in heavily-doped GaP.     

Valence and core excitations in rare gas mono- and multilayers: Production,decay, and desorption of neutrals and ions

The aim of this survey is the understanding of the dynamics of medium to high energy excitations in simple condensed systems on very short time scales. For this purpose we examine the modifications of the electronic excitations and their evolution in rare gases (mainly Ar and Kr) due to a nearby metal surface (monolayer case) or by embedding into a rare gas condensate (multilayers). Ionic excitations are shifted to lower energies compared to the gas phase by polarization of the surroundings, while neutral excitations stay constant or are shifted to somewhat higher energies. This decreases the spacing between excitonic and ionic states from both sides. Deexcitation events can be analysed by linewidths for valence excitations, and by comparison of autoionisation and Auger spectra for core excitations. For monolayers, we conclude that excitonic states are unstable relative to ionic states but nevertheless are quite long-lived. For multilayers, only minor modifications relative to the gas phase are usually found. All electronic excitations in Ar and Kr mono- or multilayers lead to desorption of neutrals; core excitations in multilayers also lead to ions and cluster ions. The probable mechanisms in all cases are discussed, and open questions are pointed out.     

Seeing many-body effects in single- and few-layer graphene: observation of two-dimensional saddle-point excitons

Significant excitonic effects were observed in graphene by measuring its optical conductivity in a broad spectral range including the two-dimensional π-band saddle-point singularities in the electronic structure. The strong electron-hole interactions manifest themselves in an asymmetric resonance peaked at 4.62 eV, which is redshifted by nearly 600 meV from the value predicted by ab initio GW calculations for the band-to-band transitions. The observed excitonic resonance is explained within a phenomenological model as a Fano interference of a strongly coupled excitonic state and a band continuum. Our experiment also showed a weak dependence of the excitonic resonance in few-layer graphene on layer thickness. This result reflects the effective cancellation of the increasingly screened repulsive electron-electron (e-e) and attractive electron-hole (e-h) interactions.     

Relaxation dynamics of electronic excitations in CaWO4 and CdWO4 crystals studied by femtosecond interferometry technique

The relaxation of electronic excitations in CdWO₄ and CaWO₄ crystals was studied using the method of time-resolved interferometry with 100-fs temporal resolution at temperatures 15–295 K. The electronic system was excited in the one-photon and two-photon regime within the excitonic band in CaWO₄ and in the electron-hole continuum in CdWO₄. Immediate trapping of charge carriers was detected under pumping in the excitonic band of CaWO₄. This result is in agreement with decay kinetics measurements with nanosecond time resolution under direct creation of excitons by 100-fs laser pulses. Fast relaxation of charge carriers followed by formation of excitons was observed in CdWO₄. The comparison with previous work allows suggesting the formation of bulk excitons and surface-perturbed excitons in the multi-photon and one-photon regime. The corresponding models of self-trapped exciton creation in tungstate crystals are discussed.     

CO dynamics induced by tunneling electrons: differences on Cu(110) and Ag(110)

The electronic current originating in a scanning tunneling microscope (STM) can be used to induce motion and desorption of adsorbates on surfaces. The manipulation of CO molecules on noble metal surfaces is an academic case that has received little theoretical attention. Here, we do thorough density functional theory calculations that explore the chemisorption of CO on Cu(110) and Ag(110) surface and its vibrational properties. The STM induced dynamics are explored after excitation of the highest lying mode, the C–O stretch. In order to give a complete account of this dynamics, the lifetime of the different CO modes is evaluated (by only including the mode decay into electronic excitations of the host surface) as well as the intermode coupling. Hence, after excitation of the stretch mode, the lower-energy modes are populated via intermode coupling and depopulated by electron-hole excitations. This study reveals the intrinsic features of the STM induced motion of CO on Cu(110) and Ag(110).     

Comment on the electron-hole droplet condensation in direct gap semiconductors

The electron-hole droplet nucleation in highly excited direct gap semiconductors is a non-equilibrium phase transition of second order. Within the framework of a Fokker - Planck approximation modifications of the thermodynamic phase diagram and the cluster distribution function are calculated. Due to the short lifetime of the electronic excitations only very small electron - hole clusters can be formed.     

Intrinsic and extrinsic luminescence of nanosize transition alumina powders

Luminescence spectroscopy in the VUV-visible range under electron-beam excitation and synchrotron radiation was applied to investigate electronic properties of alumina nanopowders, which were prepared using the combustion synthesis method. By varying reaction and post treatment conditions we were able to prepare phase pure samples and powders with mixtures of α- and γ-phases mainly. In addition to the well-known 7.6 eV luminescence of STE of α-alumina, all samples possessed complex emission bands in UV range (3–5 eV) which originate from intrinsic excitonic emissions and extrinsic electronic excitations.     

Excitonic polarons in semiconductor quantum dots

The discretization of the electronic spectrum in semiconductor quantum dots implies a strong coupling behavior between the optical phonons and the electron-hole pairs, despite the fact that a pair is electrically neutral. The excitonic polarons strongly modify the optical spectra. In particular, the ground excitonic polaron contains one or two phonon components, which leads to the existence of phonon replicas in the luminescence. The population and coherence decay times of the optical transition associated with the ground excitonic polaron are calculated.     

Multiplication of anion and cation electronic excitations in luminescent wide-gap ionic crystals

The spectra of intrinsic luminescence excitation by synchrotron radiation (6–32 eV) at 8 K have been analyzed for NaCl, KCl, RbCl, KBr, RbBr, CsBr, MgO, CaO and YalO₃ crystals. In all crystalsv (except MgO and CaO) the process of multiplication of electronic excitations (MEE) causes a sharp increase of the intensity of self-trapped exciton emission, but leads (in KBr and NaCl) to the decrease of intra-band luminescence efficiency. The analysis of the intensity ratio spectra for two components of exciton emission allows us to separate the process of secondary exciton creation by hot photolectrons (NaCl, KBr, YAlO₃). The threshold energies of excitonic and electron-hole mechanisms of MEE are compared for a number of alkali halides.     

Excitations in charge-transfer atom scattering

The electronic excitations are calculated for a tight-binding model of 25–1000 eV Na atoms scattering off W, assuming a classical trajectory for the Na atoms which interact with a single half-filled band on the substrate. The excitation spectrum consists of substrate electron-hole pairs at low energies, with a jump at the ionization threshold due to electron transfer from the Na to the W. If the Na ionization level crosses the Fermi energy beyond the range of hopping between the Na and the W substrate, the ionization probability is high. As the Na kinetic energy is reduced the ionization probability decreases, but the substrate electron-hole excitations increase in importance, and this is discussed in semi-classical terms.     

Theory of elementary excitations and the metal-insulator transition of semiconductor surfaces

The microscopic theory of density and spin response of surface systems and its application to elementary excitations is discussed. Particular emphasis is placed on semiconductor surfaces, for which the often-used jellium approximation is not valid. The discussion is based on a solution of Maxwell's equations or, formally, of the Bethe-Salpeter equation for the two-particle Green's function of the surface system. This solution is achieved in a local wave function representation and takes density fluctuations on a microscopic scale (surface profile and local-field effects parallel to the surface) into account. Many-body effects of random-phase (RPA) and electron-hole type are included. The resulting spin and density response functions present a practical scheme for a microscopic calculation of surface elementary excitations in conducting as well as non-conducting solids. As examples, the conditions for the appearance of an electronic (charge- and spin-density) instability at the surface and the coupling of the resulting charge-density wave to the lattice are studied in detail.Results of quantitative calculations of the charge- and spin-density-response function of the Si(111) surface establish the importance of including both excitonic (electron-hole) and (RPA) local-field many-body interactions. In particular, they lead to an instability of the ideal paramagnetic surface with respect to spin-density waves (SDW) with wavelength corresponding to the observed (2 × 1) and (7 × 7) superstructures. Another example deals with an a-priori calculation of the phonons and the electron-phonon interaction of the same surface system. Various results of the theory such as phonon softening due to the coupling of the charge-density fluctuations to the lattice are summarized and general aspects of the importance of many-body effects for the a-priori determination of surface structures via elementary excitations are discussed.     

Ab-initio optical properties and dielectric response of open-shell spinel zinc ferrite

In the present work, we predict the optical properties and the dielectric response spectrum of the spinel zinc ferrite Zn₂Fe₄O₈, and show in particular the impact of many-body effects on the absorption spectrum, using advanced many-body perturbation approach. The excitonic effects remarkably redistribute the spectral weights causing a red-shift of 1.6 eV of the maximum of the independent particle G ₀ W ₀?(IP-G ₀ W ₀) towards the electron-hole affected spectrum. The excitation spectrum of the zinc ferrite exhibits a low lying doubly degenerated bound dark exciton at 1.84 eV with a fully symmetric excited-state density, and a narrow optical gap setting on at 1.93 eV. We further analyse the electronic transitions and exciton density distributions giving insights to the nature of excitations. The dielectric response of Zn₂Fe₄O₈ shows a particular sensitivity to the excitations higher than the electronic band gap, however it abruptly becomes passive to the incoming electro-magnetic wave and propagates to the negative regions at high energy regimes.     

Revised fine splitting of excitons in diamond

We study low-strain synthetic high pressure, high temperature diamonds by cathodoluminescence and observe novel fine structure in the free exciton and the boron-bound exciton emission. The basic spectral structure is a doublet with DeltaE approximately 11 meV common to both exciton spectra. This resolves the previously found inequivalence of free exciton ( approximately 7 meV) and bound exciton ( approximately 12 meV) fine splitting. It is argued that for a spin-orbit interaction Delta(0) much smaller than the excitonic binding ( E(X) approximately 80 meV) and the excitonic localization ( E(loc) approximately 51 meV) at the boron acceptor, the orbital momentum and the spin of the particles constituting the electron-hole pair are recoupled to form spin singlet and triplet exciton states as the elementary excitations.     

Electronic transitions of Ar,Xe, N2, CO physisorbed on Ag(111) and Al(111)

High Resolution Electron Energy Loss Spectroscopy has been extended to study also the excitonic (low lying electronic) transitions of physisorbed rare gas atoms (Ar, Xe) and diatomic molecules (N₂, CO) on Ag(111) and Al(111) surfaces at ～20K. Electron Loss Spectra were performed using a pair of hemispherical analyzers mounted at a fixed scattering angle (90°). This spectrometer allowed high transmission in the range of 0–15eV loss energies and incident beam energies up to 2OeV. AES, LEED and UV Photoemission (HeI) were also used in situ to characterize these surfaces and to identify the adsorbed gases and delineate their absolute coverage regimes.In contrast to optical absorption experiments, we observe both, optical (dipole) forbidden and allowed electronic transitions which show vibrational line structure for condensed multilayers. By comparison to gas phase data we find only weak perturbations in the condensed state. The observed electronic excitations show changes in intensity and FWHM depending on the coverage of the adsorbed gases.The FWHM of the electronic excitations of CO and N₂ adsorbed in the monolayer regime is larger than in multilayers. Nitrogen, on both surfaces exhibits an increase from 60meV to 120meV (FWHM) whereas for CO the vibronic features are broadened out leaving peaks with FWHM of ～1eV.The intensities of the electronic losses for all gases are smaller in the first monolayer than in the second or in multilayers. At submonolayer coverage the loss intensifies due to electronic excitations are strongly reduced and no longer observable although vibrational bands and photoelectron spectra show the presence of physisorbed adsorbates.Our results will be compared to optical absorption experiments (ref.1) on similar systems and to atom-on-jellium calculations (ref.2).     

Nonequilibrium properties of electron-hole plasma in direct-gap semiconductors

The gain spectra of the electron-hole plasma recombination in CdS are investigated as a function of the excitation conditions and of the lattice temperature. From a lineshape analysis which includes such many-body effects as collision broadening, single-particle energy renormalization and excitonic enhancement, average plasma parameters are obtained. In contrast to the predictions of quasi-equilibrium theory, one finds that the electron-hole plasma does not reach a full thermal quasi-equilibrium in direct-gap materials because of the short lifetimes of the carriers. The nonequilibrium effects are shown to lead to the formation of electron-hole plasma density fluctuations. No well-defined coexistence region exists. The experimental results in the phase transition region can consistently be explained by theoretical treatments of this nonequilibrium phase transition.